The present invention relates generally to voltage followers stages and more specifically to an improved slew rate for low input current, voltage follower.
Slew rate of operational amplifiers is defined as the maximum rate of change of the output signal in response to changes in the input signal. Ideally, the amplifier output, under large signal conditions, should follow the input signal. However, limitations on the slew rate cause the amplifier output response to be slower than the rate of change of the large input signal. The greater the slew rate, the more closely the output signal follows the input signal. Various techniques have been developed to increase the slew rate of operational amplifiers.
One of the prior art techniques is described in U.S. Pat. No. 4,250,460 to Gasparik. A uniquely defined capacitance is connected in parallel to the common current source of a differential pair to increase the slew rate.
As discussed in U.S. Pat. No. 4,560,948 to Prentiss et al., the increasing of the transconductance of a stage also increases the bandwidth and slew rate.
A limiting factor on the slew rate is the output capacitance. The slew rate is proportional to the output current divided by the output capacitance. One solution to this problem is to increase the slewing current when needed such that the ratio of the output current to the output capacitance is greater during slewing conditions. This method is described in U.S. Pat. Nos. 4,636,743 to Cotreau and 4,636,744 to King et al.
Other nodes or points in other types of circuits have parasitic capacitance which limit the slew rate of the node. The output node of a voltage follower stage illustrated in FIG. 1A is an example. As in operational amplifiers, increasing the current at the node will improve the slew rate. This is not desirable in some applications. Another distinction between operational amplifiers and parasitic capacitance at other circuit nodes is that the capacitor in the operational amplifier which determines the slew rate must always have one terminal grounded to achieve compensation. Other circuit nodes may not have this limitation.
It is an object of the present invention to provide another method of increasing the slew rate of circuit nodes. Another object of the present invention is to increase the slew rate of current follower stages.
Still another object of the present invention is to provide a voltage follower stage having increased slew rate which is capable of receiving low input currents.
These and other objects are obtained by providing a correction or bootstrap circuit in a voltage follower stage so as to effectively remove the parasitic capacitance by bootstrapping the capacitance so that both sides of the parasitic capacitance follows voltage level changes. In a circuit having a first transistor wherein the input is applied to the base and the output is taken off the collector-emitter path which is connected to a current source, the bootstrap circuit is connected in parallel to the current source for causing both sides of the parasitic capacitance of the current source to follow changes of voltage at the point of connection to the emitter-collector path of the first transistor. The bootstrapping circuit generally includes a second transistor having its base connected to the emitter-collector path of the first transistor and its emitter-collector path connected to the current source. The current source itself can include a third transistor, acting as a first small current source, having its emitter-collector path connected in series with the emitter-collector path of the first transistor and a base connected to the emitter-collector path of the second transistor. The second transistors' emitter-collector path is connected to a second, larger current source by a diode and the other end of the emitter-collector path of the third transistor is also connected to the same current source. The third transistor and the diode are sized such that the second transistor carries substantially more current than the first transistor by at least ten times. A level shifter may be provided at the output of the second transistor.
The bootstrap concept may also be applied to each leg of a differential amplifier having differential voltage inputs and differential current outputs. The main current source and the diode of the second transistors in the differential amplifier are common to each leg. A bootstrap circuit may be provided for the second transistor also.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.